7. Equilibria
A section of Chemistry, 9701
Listing 10 of 344 questions
Compare and explain the relative acidities of butanoicacid, ethanol, ethanoicacid and water. > > > most acidic least acidic Three carboxylic acids, methanoic acid, HCO2H, ethanedioic acid, HO2CCO2H, and butanedioicacid, HO2CCH2CH2CO2H, are compared. Two tests were carried out on separate samples of each organic acid, as shown. The following results were obtained.  = observed change  = no observed reaction test reagents and conditions HCO2H HO2CCO2H HO2CCH2CH2CO2H observed change       Complete the table with the reagents and conditions and the observed change for a positive test. Assume these organic acids all have a similar acid strength. Each compound, HCO2H, HO2CCO2H and HO2CCH2CH2CO2H, is dissolved seperately in CDCl 3. Proton (1H) NMR and carbon‑13 (13C) NMR spectra are then obtained. Complete the table. compound number of peaks in proton NMR number of peaks in carbon-13 NMR HCO2H HO2CCO2H HO2CCH2CH2CO2H  The proton NMR spectrum of HCO2H in D2O is obtained. Describe and explain the difference observed between this spectrum and the proton NMR spectrum of HCO2H in . 1,4-dibromobutane, Br(CH2)4Br, is used in the synthesis of the dicarboxylic acid J and diamineK as shown. step 1 step 2 G step 3 H J K HO2CCH2CH2CO2H H2NCH2CH2CH2CH2CH2CH2NH2 step 4 BrCH2CH2CH2CH2Br Draw the structures of G and H in the boxes. Suggest reagents and conditions for each of steps 1 to 4. step 1 step 2 step 3 step 4  PolyamideL can be synthesised from dicarboxylic acidJ, HO2C(CH2)2CO2H, and diamineK, H2N(CH2)6NH2. Draw the repeat unit of the polymer formed in the box. Any functional groups should be shown displayed. polyamide L  
9701_s21_qp_43
THEORY
2021
Paper 4, Variant 3
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Aqueous solutions of methanoic acid, HCOOH, and propanoic acid, CH3CH2COOH, are mixed together. An equilibrium is set up between two conjugate acid–base pairs. Define conjugate acid–base pair. The pKa of HCOOH is 3.75 and of CH3CH2COOH is 4.87. Complete the equation for the Brønsted–Lowry equilibrium between the stronger of these two acids and water. + H2O + Write an expression for the acid dissociation constant, Ka, for butanoic acid, CH3CH2CH2COOH. Ka = The pKa of CH3CH2CH2COOH is 4.82. A solution of CH3CH2CH2COOHhas a pH of 3.25. Calculate the concentration, in mol dm–3, of CH3CH2CH2COOH in this solution. concentration of CH3CH2CH2COOH = mol dm–3 Define buffer solution. A buffer solution containing a mixture of CH3COOH and CH3COONa is prepared as follows. A solution of 600 cm3 of CH3COOH is mixed with 400 cm3 of 0.125 mol dm–3 CH3COONa. The buffer solution has pH 5.70. The Ka of CH3COOH is 1.78 × 10–5 mol dm–3. Calculate the initial concentration, in mol dm–3, of CH3COOH used. concentration of CH3COOH = mol dm–3 A fuel cell is an electrochemical cell that can be used to generate electrical energy by using oxygen to oxidise a fuel. Methanoic acid, HCOOH, is being investigated as a fuel in fuel cells. When the cell operates, HCOOH is oxidised to carbon dioxide. The half‑equation for the reaction at the cathode is: O2 + 4H+ + 4e– 2H2O. In this fuel cell, the overall cell reaction is the same as that for the complete combustion of HCOOH. Deduce the half‑equation for the reaction at the anode. Calculate the volume, in cm3, of oxygen used when a current of 3.75 A is delivered by the cell for 40.0 minutes. Assume the cell operates at room conditions. volume of oxygen = cm3 , ,
9701_s24_qp_42
THEORY
2024
Paper 4, Variant 2
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Questions Discovered
344